1. Field of the Invention
The invention relates to clock calibration techniques. More particularly, the invention relates to a process for determining clock drift in oilfield operations.
2. Background Art
In the oil-drilling and exploration industry, many types of sensors are used to measure phenomena related to subsurface physical properties (e.g. density, conductivity, or porosity) to evaluate subsurface conditions, commonly referred to as well logging. Most of these measurements can be performed either after a borehole has been drilled, using a wireline tool, or simultaneously with the drilling of the borehole, i.e., logging-while-drilling (LWD) or measurement-while-drilling (MWD). LWD/MWD measurements are generally taken with tools mounted within drill collars forming part of a drill string.
One type of LWD/MWD technique uses sound waves, also referred to as seismic or acoustic waves to measure subsurface properties. These seismic systems generally entail a seismic source, sensors, and a memory and calculating device for storing and processing the received seismic signals. Conventional seismic sources generate a physical disturbance that produces acoustic or seismic signals that propagate through the subsurface medium (formation or water) and are detected by a remote acoustic sensor. The acoustic sensors (hydrophones or geophones) may be located in a drill string for LWD/MWD measurements, or in a casing segment for monitoring operations, and typically in the vicinity of the source. The geophones or hydrophones in the drill string transmit the detected signals to a memory and calculation unit that processes the signals. U.S. Pat. Nos. 6,308,137, 5,585,556, 5,130,949, 5,144,589, 6,430,508, and 4,363,112 generally describe oilfield seismic measurement techniques.
The acoustic signals or waves produced by the seismic source are periodic vibrational disturbances resulting from the acoustic energy propagating through the medium. These signals are detected by the hydrophones or geophones and typically characterized in terms of their frequency, amplitude, and speed of propagation. The transit or arrival times of the acoustic signals through the subsurface medium provide useful information of the subsurface properties. Clocks or chronometers are used to determine the elapsed time between the initial source firing and the receipt of the signals detected at the sensor. Application of standard physics principles using signal speed, elapsed time, and distance, allows one to determine the subsurface parameters in the seismic measurement. As known in the art, the propagation speed of an acoustic signal is influenced by the medium and must be taken into account in seismic measurements.
One way of accounting for the influence of the medium on seismic measurements is known as a “checkshot” measurement. A checkshot or test signal is transmitted a known distance through the medium and the signal travel time is used to determine the signal speed. The elapsed travel time of the acoustic signal is typically determined using a clock coupled to the source and synchronized with a clock coupled to the remote sensor. In this manner, the two clocks, along with a processing device, make possible a precise calculation of the transit or arrival time of the seismic signal between the source and the remote sensor.
The use of independent clocks to determine the elapsed signal travel time has its drawbacks. In conventional drilling and monitoring operations, the measurement apparatus are typically disposed in subsurface mediums for extended periods of time. The longer the period of sustained subsurface measurements, the greater the influence of the natural drift between the source clock and the sensor clock becomes on the seismic measurements unless the clocks are calibrated back into synchronization. The degree of drift between the clocks is affected by factors including calibration errors, clock accuracy, and the clock housing (temperature control, shock resistance, etc.). Various approaches have been developed to address clock drift. U.S. Pat. Nos. 6,078,868, 6,400,646, 6,131,694, 4,281,403 and U.S. Published Pat. App. Ser. No. 2002/0,060,952A1 describe various approaches to compensate for clock drift.
There remains a need for improved techniques to determine clock drift and to compensate for such drift or calibrate the clocks.